The use of hollow-fiber membranes for separation of mixtures of liquids and gases is well-developed and commercially very important art. Such membranes are traditionally composed of a homogeneous, usually polymeric composition through which the components to be separated from the mixture are able to travel at different rates under a given set of driving force conditions, e.g. trans-membrane pressure and concentration gradients. Examples are the desalination of water by reverse osmosis, separation of water/ethanol mixtures by pervaporation, separation of hydrogen from refinery and petrochemical process streams, enrichment of oxygen or nitrogen from air, and removal of carbon dioxide from natural gas streams. In each separation, the membranes must withstand the conditions of the application, and must provide adequate flux and selectivity to be economically attractive. The use of hollow fibers is recognized to have advantages over flat film or planar membranes due to the large membrane surface area for separation within a specific volume of apparatus. The success of polymeric hollow fiber membranes has in part been due to the ability to produce fibers of extremely small diameter—in some cases, the diameter of a human hair (˜80 μm). The ability to utilize small-diameter fibers allows extremely high module surface areas, which allows processing high volumes of fluid for each membrane module.
In certain applications where high chemical resistance and operation at high temperature and pressure are desired, polymeric membranes have not been suitable because of degradation of membrane performance during operation. Inorganic or ceramic membranes have been successfully made in flat or planar shapes and large cylindrical tubes (>1 cm diameter), but have had limited commercial success because of their relatively low surface area compared to small-diameter hollow fibers. Production of small-diameter ceramic hollow fibers has been problematical with respect to strength of the precursor fiber (sometimes referred to as “green” fiber) and the final fiber after sintering.
Such hollow fibers are typically made from a suspension of inorganic particles in a liquid medium with a suitable binder to form a paste, which is subsequently extruded through an annular die to form a precursor hollow fiber. After removal of the liquid dispersion medium, the precursor fiber is sintered at elevated temperature to consolidate the individual particulate structure into a micro-porous structure.
For the production of small-diameter inorganic fibers, it has been found to be beneficial to incorporate a polymeric binder in the paste to strengthen the nascent fiber. The polymer is typically soluble in the liquid medium of the paste. After the paste is extruded to form a nascent hollow fiber, the polymer solution in the interstices between the inorganic particles is coagulated to solidify the polymer by passing the nascent fiber into a liquid bath containing a coagulating fluid; alternatively, the liquid can be removed by evaporation to solidify the polymer. The resulting polymeric/inorganic precursor fiber has considerably greater strength and ductility than in the absence of a polymeric binder.
U.S. Pat. Nos. 4,175,153, 4,222,977, 4,268,278, and 4,329,157 disclose a process to make a polymeric/inorganic precursor for inorganic hollow fibers via extrusion of a mixture of an inorganic material uniformly dispersed in a polymer solution. The polymer solution comprises a fiber-forming organic polymer dissolved in a suitable solvent.
U.S. Pat. No. 5,707,584 discloses a process to make a polymeric/inorganic precursor for ceramic hollow fibers by melt extruding a paste consisting of a thermoplastic polymer binder system with a ceramic powder through a spinneret. The preferred polymer composition is polyethylene/vinyl acetate copolymer mixed with various plasticizers.
U.S. Pat. No. 6,261,510 discloses a process to make a polymeric/inorganic precursor for ceramic hollow fibers by extruding a paste consisting of a water-soluble polymer, typically methylcellulose in water solvent, through a spinneret at room temperature.
U.S. Pat. No. 6,492,290 discloses polymeric/inorganic precursors for ceramic ion-conducting planar membranes by extruding a paste consisting of a mixture of inorganic metallic oxides dispersed in solution of polyvinylbutyral in a suitable solvent.
U.S. Patent Application 2006/0154057 A1 discloses a process to make a polymeric/inorganic precursor for inorganic hollow fibers via extrusion of a mixture of an inorganic material dispersed in any suitable thermoplastic polymer, or an acrylate-based polymer system that can be cross-linked after extrusion.
World Patent Application WO2007/007051 discloses a process to make a polymeric/inorganic precursor for inorganic hollow fibers via extrusion of a mixture of an inorganic material dispersed in a polymer solution, typically comprised of polyethersulfone and solvent.
Liu and Gavalas (J. Membrane Science 246 (2005) 103-108, Elsevier) describe making a polymeric/ceramic hollow fiber precursor by spinning a dispersion of perovskite in a polysulfone solution into an aqueous coagulation bath, which imparts some degree of asymmetry in the fiber wall. The precursor fiber was sintered at 1190° C. to form a ceramic hollow fiber.
EP1547672 discloses a ceramic hollow fibre membrane module.
U.S. Pat. No. 4,348,458 discloses methods of manufacturing coiled ceramic hollow fibres.
WO2003051495 discloses methods of manufacturing ceramic hollow fiber membrane modules.
An object of this invention is to produce small-diameter precursor polymeric/inorganic hollow-fibers that exhibit the desired processability and strength during manufacture with the desired microstructure morphology that, after sintering, provides a suitably effective separation membrane.